Various versions of inductive links and associated methods have been described in the literature and utilized for wireless data and/or power transmission for use with implantable medical devices (IMDs). Well-known examples of such inductive links include those used with cochlear implants and visual prostheses, which typically involve sending a large volume of data from external artificial sensors to the IMD. The communication direction in these types of IMDs (from an external source to the implantable device) may be referred to as a “downlink.” Other types of IMDs, such as invasive brain-computer interfaces (iBCI), in which large amount of information is collected from the central nervous system and sent to outside of the body for further processing may predominantly utilize the “uplink” communication direction. Most radio frequency identification (RFID) applications also utilize inductive links to energize the ultra-low power battery-less RFID tags and interrogate the tags to read their stored information.
Some of the challenges involved in designing transcutaneous data and power transmission links relate to the extremely limited space and power available to the IMD for establishing a wideband and robust connection. One goal is to achieve the highest possible data rates at the lowest possible carrier frequencies within the frequency bands of interest. However, because of the electro-magnetic field absorption in the tissue, which increases at a rate of carrier frequency squared, f2, the competing goals of low carrier frequency and high data rate often rule-out a majority of established wideband wireless protocols, such as Bluetooth or WiFi, which operate at 2.4 GHz.
Certain dedicated medical communication frequency bands exist, such as the Medical Implant Communication Service (MICS), operating in the 402˜405 MHz range, but typically can only provide a limited data bandwidth of approximately 300 kHz. Therefore, inductive coupling within 1-20 MHz band is the most common method that has been utilized for establishing wideband data telemetry and efficient power transmission to neuroprostheses.
The majority of modulation techniques that have been devised for near-field data transmission modify the amplitude, frequency, or phase of a sinusoidal carrier signal based on the data to be transferred across the inductive link. For example, amplitude shift-keying (ASK), frequency shift keying (FSK), load shift keying (LSK), and binary/quadrature phase shift keying (BPSK/QPSK) are examples of such conventional methods. The use of a power carrier signal along with these methods was attractive in the early IMDs because the same inductive link could be used for both power and data transmission.
In high-performance IMDs that require wider bandwidth, however, a separate power carrier from the data carrier is typically preferred because increasing the frequency of the high amplitude power carrier can lead to unsafe temperature elevation due to excessive power loss in the tissue. To achieve high power transfer efficiency (PTE) and high data rate, a high frequency carrier (>50 MHz) may be required for the data link, while it may be beneficial to keep the power carrier frequency below about 20 MHz. This has led to the use of dual-carrier power/data links with each carrier linking a separate pair of coils.
A major challenge in dual-function (power & data) inductive link designs is the cross-coupling between the two pairs of power and data coils. Part of the challenge is that to conserve space, the coils in the IMD need to be miniaturized and co-located inside the IMD. However, a strong power carrier signal can interfere with and/or dwarf the weak data signal on the receiver (Rx) side and make data recovery quite difficult, if not impossible. While certain innovative coil designs can help with reducing cross-coupling, it may still be necessary to electronically filter out the power carrier interference at the receiver (Rx) input at the cost of adding to the power consumption and complexity of the IMD. Moreover, achieving high data rates via traditional modulation schemes often requires power consuming frequency-stabilization RF circuits, such as phase-locked loops (PLL), which can add to the size, complexity and power requirements of the IMD.